1. Field of the Invention
This invention is related to a power supplying apparatus. More specifically, this invention relates to a power supplying apparatus for a print head comprised in a thermal inkjet printer.
2. Description of the Prior Art
At the present day, thermal inkjet printers are prevalent printers. Monochrome inkjet printers generally include a print head for providing black ink; color inkjet printers generally further include several print heads for providing inks of basic colors. Each black or color print head has a plurality of nozzles, respectively. Nozzles are the components that actually eject inks. Most inkjet printers are capable of providing a plurality of printing modes; each printing mode corresponds to a respective print resolution and a respective print quality. According to different settings of print resolution, the number of nozzles being used changes. That is to say, a printer controls different nozzles to act based on the printing mode selected by a user. A complete character or figure is formed with lots of tiny ink drops ejected by nozzle. Since each character and each figure are different from other, the number of nozzles being used is uncertain.
The working principle of thermal inkjet printers is briefly explained as follows. When a certain nozzle is assigned to act, a voltage impulse is provided to a heater in the nozzle so that the heater can generate heats rapidly. A small bubble is created and gradually grows up in the nozzle due to the heats. An ink drop near the output of the nozzle is then pushed out by the bubble so that the ink drop is ejected to a paper and forms a small ink point. After the voltage impulse is ended, the heater cools down, the bubble disappears, and the other ink in the nozzle shrinks back to the inner side of the nozzle. Please refer to FIG. 1. FIG. 1 shows the typical connecting relation between a power source 10 and the nozzles (111, 112, and 113) in a print head 11. The power source 10 is used for providing voltage impulses to the nozzles. Each nozzle respectively includes a heater (111A, 112A, and 113A) and a switch (111A, 112A, and 113A).
The nozzle 111 is taken as an example. When the switch 111 B is opened, the heater 111A is not conducted to the ground. Therefore, there is no current flowing through the heater 111A and no voltage drop is formed between the two ends of the heater 111A. If the nozzle 111 is assigned to act in certain operation, the switch 111B is closed so that the heater 111A is conducted to the ground through the switch 111B. Subsequently, the power supply 10 provides current to the nozzle 111 and there is a voltage drop formed between the two ends of the heater 111A. An ink drop near the output of the nozzle 111 is then pushed out by the bubble generated by the heater 111A to form an ink point.
FIG. 2 further shows the schematics of the nozzles in actual applications. As shown in FIG. 2, each switch of the nozzles can be composed of a sub-switch (111C, 112C, 113C) and a MOS device (111D, 112D, 113D). When the sub-switch 111C is closed, the gate of the MOS device 111D is connected to the power supply 10 and the MOS device 111D is then conducted. In this way, the heater 111A can be conducted to the ground through the MOS device 111D.
Each heater, each switch, and even the connecting wires between the power supply 10 and the print head 11 are all loadings for the power supply 10. As known to people skilled in the art, the output impedance of an ideal voltage supply is zero, thus, regardless of the value of loadings, an ideal voltage supply can provide a constant output voltage. However, in actual situations, the output voltage provide by the power supply 10 varies with different loadings.
As shown in FIG. 1, if the nozzle 111 and nozzle 112 are assigned to act simultaneously, the heater 111A and the heater 112A are parallel. The equivalent loading formed by the two parallel heaters is half of the loading formed by a single heater (assuming the resistances of the heater 111A and the heater 112A are the same). Since the number of nozzles being used is uncertain under various conditions, the loading formed by the nozzles also changes for the power supply 10. Therefore, the voltage drop generated between the two ends of a heater is inevitably affected by the amount of loadings.
Presently, most inkjet printers use constant voltages to drive print heads. Once the voltage is affected by different loadings and connecting wires, the voltage supplied to the heaters might be insufficient or too much so that the ink drops in nozzles cannot be ejected or too big. Thus it shows loading problem does decrease printing quality of inkjet printers.
Please refer to FIG. 3. FIG. 3 shows the other prior art including two power supplies. The first power supply 20A is used for providing voltages to the heaters; the second power supply 20B is used for selectively providing voltages to the gates of the MOS devices. The main difference between FIG. 3 and FIG. 2 is that the voltage values for heaters and switches are different; thus, two power supplies are required. The prior art shown in FIG. 3 also has the aforementioned problem of voltage variations.